Method of making a high melting point solder ball coated with a low melting point solder

ABSTRACT

A method of forming a solder ball structure comprising the steps of 1) obtaining a first object having at least one high melting point solder ball, each solder ball having a corresponding BLM containing a low melting point solder, each solder ball having an outer surface, top and bottom, each BLM having a top and bottom, the bottom of each solder ball in contact with the top of the corresponding BLM over a portion of the outer surface, the bottom of each BLM in electrical communication with the first object; 2) obtaining a template having a first surface; 3) applying a disjoint area of low melting point solder paste to the first surface of the template; 4) interacting the top of each solder ball and the solder paste; and 5) reflowing each solder ball and disjoint area of solder paste while each solder ball and disjoint area of solder paste are interacted.

RELATED APPLICATIONS

This invention is related to copending U.S patent application Ser. No.08/822,461 entitled "High Melting Point Solder Ball coated with a LowMelting Point Solder," and copending U.S. patent application Ser. No.08/882,458, entitled "Method for Establishing Electrical Communicationbetween a First Object having a Solder Ball and a Second Object," bothassigned to the present assignee and filed on the same day. Each of theabove identified applications is incorporated by reference in itsentirety.

FIELD OF THE INVENTION

This invention is related to the field of semiconductor packaging. Morespecifically, the invention relates to a method for creating solderballs that provide a reliable electrical and mechanical connectionbetween a first object containing solder ball structures and a secondobject.

BACKGROUND OF THE INVENTION

The industry has moved away form the use of pins as connectors forelectronic packaging where the pins have a high cost of fabrication,unacceptable percentage of failed connections which require rework,limitations on I/O density and have the electrical limitations of therelatively high resistance connectors. Solder balls are superior to pinsin all of the above features as well as being surface mountable, whichhas obvious implications given the increasingly small dimensions in theforefront technologies today.

Solder mounting is hardly a new technology, itself. But, the needremains to improve the solder systems and configurations in electronicstructures. The use of solder ball connectors has been applied to themounting of integrated circuit chips using the C-4 (controlled collapsechip connection) technology since the method and structure were firstdescribed and patented in U.S. Pat. Nos. 3,401,126 and 3,429,040 toMiller et al., which are assigned to the present assignee. A myriad ofsolder structures have since been proposed for the mounting of IC chips,as well as for interconnection of other levels of circuitry andassociated electronic packaging.

Surface mount technology has gained acceptance as the preferred means ofjoining electronic devices together, particularly in high-end computers.As compared to more traditional pin connector methods, where a pinmounted to the backside of a ceramic module is thrust through a hole inthe board, twice the number of modules can be placed at the same boardarea. Other advantages such as smaller component sizes, greater I/Odensities, lower electrical resistance, decreased costs, and shortersignal paths have prompted the industry migration to surface mounttechnology.

A myriad of solder structures have been proposed for the surfacemounting of one electronic structure to another. Typical surface mountprocesses form the solder structures by screening solder paste onconductive, generally metallic, pads disposed on a surface of a firstelectronic structure, or "substrate". A stencil printing operation isused to align the contact mask to the pads. The solder paste areas onthe substrate are aligned to and placed on corresponding pads on asecond electronic structure, or "board". In some processes, solder pastemay alternatively or additionally be screened on the board pads. Afterplacement, the substrate and board go through a reflow operation to meltthe solder paste and create a solder bond between the corresponding padson substrate and board.

Other known surface mount technologies use solder balls rather than asolder paste to provide the solder structures. By using solder balls, amore exact and somewhat greater quantity of solder can be applied thanthrough screening. The solder balls are aligned and are held to thesubstrate and melted to form the solder joint on the conductive pads. Asbefore, the substrate with the newly joined solder balls is aligned tothe board. The solder balls are then reflowed to form a good solder bondbetween substrate and board.

However, both the solder paste and solder ball surface mount techniquessuffer when the density of the pads increase. A certain quantity ofsolder must be maintained to assure a reliable solder joint. As therequired quantity of solder becomes large relative to the pad spacing,solder bridging between non corresponding conductive pads becomes aproblem. The bridging problem is accentuated by the greater solderamount which is molten during the reflow process.

However, the manufacture of a solder joint using both solder paste andsolder balls has proven difficult. Solder balls are difficult to alignand handle during the reflow process. Different methods using vibration,brushing and vacuum in association with an alignment plate have beenproposed for dealing with solder balls alone. The addition of the solderpaste further complicates the process. Many problems were encounteredmaintaining the solder ball centrality with respect to each other and onthe substrate, even to the extend that the solder balls were missingentirely. With the misalignment of the solder balls, bridging betweenadjacent pad sites become a problem. Good physical contact between thesolder balls, solder paste and substrate must be assured whilesimultaneously preserving the alignment between substrate pads andsolder joints. Process time mushroomed as the number of process checksincreased.

One module that can be manufactured with a combination solderball/solder paste connection is a ceramic ball grid array (CBGA). CBGAmodules are preferred in some situations because a higher density ofI/Os can be packaged per unit area as compared with perimeter leadedcomponents. Additionally, CBGAs offer other advantages. CBGAs are lesssusceptible to damage while handling and there tends to less inductionnoise than pin-in-hole devices.

However, there are some potential drawbacks to using CBGAs. The solderjoints are under the CBGAs. Any repairs necessitate the complete removalof the CBGA module. It is difficult to inspect the I/O solder joints byconventional means. The repair process can be very time consuming andexpensive. To minimize the need for repairs CBGA users can invest in asolder paste volume measurement tool to assure that every pad has thecorrect amount of solder. The use of such a tool can increase assemblycycle time.

A popular method of attachment currently in use requires that the solderpaste be applied to the circuit card. The solder paste volume is thenmeasured. The CBGA module and the circuit card are brought into contactthe solder paste is reflowed.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a solderball structure that accurately supplies a predetermined amount ofsolder. It is another object of the invention to provide a high meltingpoint solder ball with an low melting point coating. It is yet anotherobject of the present invention to provide a solder ball structure wherea high melting point solder is encapsulated by an low melting pointsolder coating. It is still another object of the present invention toprovide a collapsible solder ball structure with a low melting pointcoating. It is further an object of the present invention to provide asolder ball structure that supplies specific volumes of solder. It isstill yet another object of the present invention to provide a methodfor marking a solder ball structure that supplies a specific volume ofsolder. In accordance with the above listed and other objects wedisclose and claim a method of forming a solder ball structurecomprising the steps of 1) obtaining a first object having at least onehigh melting point solder ball, each solder ball having a correspondingBLM containing a low melting point solder, each solder ball having anouter surface, top and bottom, each BLM having a top and bottom, thebottom of each solder ball in contact with the top of the correspondingBLM over a portion of the outer surface, the bottom of each BLM inelectrical communication with the first object; 2) obtaining a templatehaving a first surface; 3) applying a disjoint area of low melting pointsolder paste to the first surface of the template; 4) interacting thetop of each solder ball and the solder paste; and 5) reflowing eachsolder ball and disjoint area of solder paste while each solder ball anddisjoint area of solder paste are interacted.

BRIEF DESCRIPTION OF THE DRAWING

These and other features, aspects, and advantages will be more readilyapparent and batter understood from the following detailed descriptionof the invention, in which:

FIG. 1 is a cross sectional view of a substrate with a solder ball andBLM attached;

FIG. 2 is a cross sectional view of the contacting step of the method ofthe present invention;

FIG. 2a is a cross sectional view of a preferred embodiment of thecontacting step of the method of the present invention;

FIG. 2b is a cross sectional view of an individual coated solder ballstructure of an embodiment of the present invention;

FIG. 3 is a cross sectional view of the contacting step of analternative method of the current invention;

FIG. 3a is a cross sectional view of a preferred embodiment of thecontacting step of the method of the present invention;

FIG. 4 is a cross sectional view of the a coated solder ball structureof an alternative embodiment of the present invention;

FIG. 5 is a cross sectional view of an implementation of an alternativeembodiment of the present invention; all in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can be used for connecting solder joints to anyparts utilizing surface mount technology. In the packaging area, thereare a plethora of electronic structures which require connection toother similar electronic structures or to other levels of package. Newand creative connection protocols have necessitated the development of anew and different solder ball structures, including new collapsiblesolder ball structures.

By solder ball it is meant that a predetermined shape of a admixture ofmetals has been formed by any means known in the art and at least onereflow of the admixture has occurred. The admixture is typically of leadand tin. The shape of the admixture is left to the discretion of theuser. It should be noted that according to the method and structure ofthe present invention a solder ball does not have to be spherical. Fordescriptive purposes only, the multitude of possible geometries will bereferred to generally as solder balls.

In the present invention, a high melting point, HMP, solder ball isencapsulated by a low melting point, LMP, solder. The terms low meltingpoint, LMP, and high melting point, HMP, are not terms having specifictemperatures associated with them. Examples of materials which aresuitable include a eutectic solder of 37/63 weight percent Pb/Sn for theLMP material and a non-eutectic solder composition of 90/10 weightpercent Pb/Sn for the HMP material. There are a wide range of materialswhich would be suitable for the subject invention, many of which arerecited throughout the art with reference to solder connections.

Referring to the figures in general and FIG. 1 in particular, a methodof making the present invention is shown. As shown in FIG. 1, the bottomof a solder ball, 2, is in contact with a BLM, 3, containing a LMPsolder (LMP BLM). By BLM it is meant a ball limiting metallurgystructure. The BLM facilitates the electrical and physical connection ofthe admixture of solder with the first object. The BLM can be anintegral part of the first object or formed on the first object. For thepurposes of this invention, the BLM can be formed by any means known inthe art as long as the necessary electrical and/or physical connectionbetween the first object and the admixture of solder exists. It isnecessary that at least a portion of the BLM comprise a LMP solder. TheBLM, 3, may contain other components necessary to ensure properelectrical communication between the first object, 1, and the solderball, 2. The BLM, 3, is in electrical communication with a firstobject, 1. The BLM can be formed and/or deposited by any means known inthe art such that it can function with the first object. The firstobject, 1, is usually a substrate but the method and structure of thepresent invention are not limited by the composition of the firstobject, 1. For descriptive purposes only, the first object, 1, willhereafter be referred to as a substrate.

As shown in FIG. 2, the substrate, 1, containing the LMP BLM, 3, and HMPsolder ball, 2, is brought into contact with a disjoint area of solderpaste, 4, on a second object, 5. The shape of the disjoint area of LMPsolder paste, 4, is determined by the user. A requirement of the subjectinvention is that the reflow of the LMP BLM and LMP solder paste occurwithout a significant portion of the HMP solder ball melting.

A reflowing is accomplished while the solder ball 2 is in contact withthe solder paste, 4. The reflowing can be accomplished by any meansknown in the art. The reflowing should occur at a temperature that ishigh enough to melt the LMP solder paste, 4, and the LMP solder BLM, 3,without melting a significant portion of the HMP solder ball, 2.

A reflow will give the final form of the structure of one embodiment ofthe present invention (See FIG. 2b) where the HMP solder ball isessentially encapsulated in the LMP solder. The reflowing will takeplace at a temperature that will cause only the LMP solder in the BLMand the LMP solder paste to melt without a significant portion of theHMP solder ball melting.

In a preferred embodiment, the solder ball diameter is 32 to 36.5 mils.In a more preferred embodiment the BLM and the solder paste would be aeutectic solder. In a preferred embodiment, the reflowed solder will beone homogeneous unit once it cools.

In a preferred embodiment, the solder paste volume equals ##EQU1## WhereD=diameter of the solder ball plus the desired coating thickness, and

d=diameter of the solder ball

and the solder paste would be stencilled on the second object. Thesolder paste volume then would be measured. Also, in a more preferredembodiment the volume of the solder paste would be at least about 8689and at most about 11200 cubic mils and the solder paste volume would bemeasured using a laser tool. The height and diameter of the disjointarea of solder paste would also be controlled by the user. The diameterof the disjoint area of solder paste would be at least about 30 and atmost about 34.5 mils. The height of the disjoint area of solder pastewould be at least about 9 and at most about 16 mils. In a most preferredembodiment, the solder paste volume would be about 11,000 cubic mils,the diameter would be about 34 mils and the height would be about 12mils and the solder paste would be measured using a Cybersentry™ lasertool.

In a preferred embodiment, the second object, 5, would be a platecomprising high temperature glass or aluminum. In a more preferredembodiment the plate would be thin and flat. In an even more preferredembodiment, see FIG. 2a, the plate, 5, would be coated with anon-wettable film, 7, that is heat resistant up to at least about 260°C.

Examples of films, 7, include a polyimide tape like Kapton™ or afluorocarbon copolymer like Teflon™. In a most preferred embodiment thesecond object would be a thin, flat, high temperature resist, glassplate with a Kapton™ coating.

In a preferred embodiment, the reflowing would be accomplished at atemperature of at least about 200° C. and at most about 205° C. Also inthe preferred embodiment, the reflowing would be continued for at leastabout 20 seconds and at most about 30 seconds.

FIG. 2b shows a final structure of the more preferred embodiment, 20,after the reflowing, where the BLM and the solder paste are botheutectic. The HMP solder ball, 2, is enclosed in the LMP solder ballcoating, 6. The coating, 6, is formed from the reflowed BLM and solderpaste. The coating, 6, is present over at least the portion of thesolder ball surface not in contact with the corresponding BLM. In apreferred embodiment there would be no delineation between the portionof the coating, 6, that was contributed by the LMP solder paste, 4, andportion contributed by the LMP solder containing BLM, 3. In a preferredembodiment the thickness of the coating is 2.5 mils. Also in thepreferred embodiment, the HMP solder ball is comprised of 90% tin and10% lead in admixture.

FIG. 3 shows an embodiment of the present invention where a plurality ofcoated solder ball structures, 20 in FIG. 2b, would be formed at thesame time. The individual HMP solder balls, 2, are similar in form tothose shown in FIG. 1, where the BLM contains a LMP solder. The HMPsolder balls, 2, contact the disjoint areas of LMP solder paste, 4. Areflow is then accomplished while the solder balls, 2, are in contactwith the solder paste, 4. The after the reflow the structure as shown inFIG. 4 should exist where a LMP solder coating, 6, encapsulates each HMPsolder ball, 2. The coating, 6, is present over at least the portion ofthe solder ball surface not in contact with the corresponding BLM.

When performing the method identified in FIG. 3 there are a number offactors that can influence the final size and shape of the coated solderball structure in the alternate embodiment. Some of the factors wereintroduced in the discussion of the method given above to form thestructure of FIG. 2b. The factors include, but are not limited to, thesize and composition of the solder ball, the size and shape of thesolder paste, the choice of materials for the second object, and thereflow temperature. The parameters previously given for the preferredembodiment described in FIGS. 2, 2a and 2b which pertain to the solderball, solder paste and the BLM also apply in a corresponding manner inthis alternative embodiment.

In a preferred embodiment of the alternative embodiment, there isanother factor that can be controlled. When coating solder balls, 2, asshown in FIG. 3a, it can desirable to control the distance between thetop of the solder ball and the surface of the second object, A, toachieve a consistent solder ball coating thickness. The shape of thefinal coated structure can be effected by the distance, A. The distancecan be controlled by monitoring the distance, A, directly. An indirectmeasurement of A can also be achieved by monitoring the distance betweenthe surface of the substrate, 1a, and the surface of the second object,7a, if the distance between the substrate and the top of the solder ballis known or predictable. The substrate and the second object could bemoved toward each other a predetermined distance such that the distanceA is then maintained. In a most preferred embodiment the distance, A, is7 mils.

In a preferred embodiment, standoffs, 8, would be employed to ensurethat the distance A was maintained as shown in FIG. 3a. The standoffsmay be comprised of any material able to withstand temperatures of atleast about 205° C. The standoffs would be located in an area where theywould not interfere with the step of contacting the solder balls and thesolder paste and could be attached to either the substrate or the secondobject prior to the contacting. Alternatively, the standoffs could beattached to neither the substrate nor the second object and could beinserted between the substrate and the second object by the same actionthat brings the substrate and the second object into contact with eachother. In a more preferred embodiment, the standoffs would be attachedto the second object.

An example of the final structure, 20, that is achieved by thealternative embodiment shown above is shown in FIG. 4. The compositionparameters for the HMP solder ball coating, 6, and the HMP solder ball2, are the same as those for the solder ball shown in FIG. 2b. In apreferred embodiment there would be no delineation between the portionof the coating, 6, that was contributed by the LMP solder paste andportion contributed by the LMP solder containing BLM. In a preferredembodiment the substrate would be a module. In a more preferredembodiment the module would be a ceramic ball grid array.

The structure of the present invention can be used to attach an objecthaving the solder ball structure to a second object. The connectionshould at least be electrical. The connection could also be physical ormechanical. There are any number of potential applications. A LMP coatedHMP solder ball can be used for direct chip attach to a circuit cards.Also, LMP coated micro HMP solder balls can, be used on a chip attachedto another substrate such as circuitized ceramic, polyimide, Teflon™ orplastic. The structure of the present invention can also be used insituations where coplanarity of the two objects being connected is aconcern. Where one of the two objects to be connected can potentially bewarped by more than 2 mils, the connection can be more effective if thesolder balls are collapsible, as in the present embodiment.

For example, a circuit card may have more than a 2 mil warpage, for anynumber of reasons. The warpage may occur during manufacturing or it maybe the result of heat fluctuations during a repair process. Anembodiment of the structure of the present invention may be used whenthere is warpage greater than about 2 mils. in the second object. Thethickness of the coating on the solder ball can be adjusted depending onthe actual or predicted warpage of the second object.

FIG. 5 shows an implementation of the structure of FIG. 4. A substrate,1, with a plurality of distinct coated solder balls, 20, thereon isbrought into contact with a second object, 10. It should be noted thatthe structure of the current invention is still operable if none of thecoated solder balls are in contact with the circuit card, as long asthey are proximally situated. By proximally situated it is meant thatthe substrate and the circuit card are no more than a maximum distancefrom each other. The maximum distance is a function of ball size, pitchbetween the balls and the coating thickness. The formula is maximumdistance (Cd)= ##EQU2## where P=the pitch between the solder balls andDmax=the maximum diameter of the solder ball. Cd is also equal to themaximum coating thickness. The present invention is operable when themaximum distance between the top of any individual coated solder balland its corresponding pad is at most about 3.5 mils.

In the current example, shown in FIG. 5, the second object is a circuitcard. The circuit card, 10, has a plurality of solder ball attachmentpoints, 9. There is one attachment point, 9, for each solder ballstructure, 20, connection desired. In the current example the attachmentpoint, 9, is a copper pad which connects to the circuit card circuitry.It can be seen that due to warpage of the circuit card, 10, at least oneof the attachment points, 9, may not be in contact with thecorresponding coated solder ball, 20. A reflow is then accomplished. Ina preferred embodiment, the reflow is accomplished while at least one ofthe coated solder balls, 20, is in contact with the correspondingattachment point, 9. In a more preferred embodiment a majority of thecoated solder balls, 20, are in contact with the correspondingattachment point. Also, in a preferred embodiment, each attachmentpoint, 9, is coated with a material to facilitate the reflow. In a morepreferred embodiment, each attachment point is coated with a watersoluble or no-clean flux paste. In a most preferred embodiment, thereflow temperature is at least about 200° C. and at most about 220° C.,the coating is 2.5 mils thick and the solder ball has a 35 mil diameterand a majority of the coated solder balls are in contact with thecorresponding water soluble flux coated attachment points.

While the invention has been described in terms of specific embodiments,it is evident in view of the foregoing description that numerousalternatives, modifications and variations will be apparent to thoseskilled in the art. Thus, the invention is intended to encompass allsuch alternatives, modifications and variations which fall with thescope and spirit of the invention and the appended claims.

We claim:
 1. A method of forming a solder ball structure comprising thesteps of:a) obtaining a first object having at least one high meltingpoint solder ball, wherein at least one of said high melting pointsolder ball has a corresponding ball limiting metallurgy structure (BLM)having a top and bottom and containing a low melting point solder, andwherein said solder ball has an outer surface, a top and a bottom andwherein the bottom of said solder ball in contact with the top of thecorresponding BLM over a portion of the outer surface and wherein thebottom of said BLM in electrical communication with the first object; b)obtaining a template having a first surface; c) applying at least onedisjoint area of low melting point solder paste to the first surface ofthe template; d) engaging the top of each at least one solder ball andthe solder paste; and e) reflowing the solder ball and the disjoint areaof solder paste while the solder ball and the disjoint area of solderpaste are interacted, wherein the high melting point solder ball iscoated with the low melting point solder over substantially all of thesurface of the high melting point solder ball not in contact with theBLM; and f) disengaging the template from the solder ball structure. 2.The method according to claim 1 wherein the template comprises materialwith a melting point that is substantially higher than the melting pointof the solder paste.
 3. The method according to claim 2 wherein thematerial is selected from the group consisting of glass and alumina. 4.The method according to claim 3 wherein the material is glass.
 5. Themethod according to claim 1 further comprising a template coatingcomprising a non-wettable film that is heat resistant to at least about260° C., and wherein the coating is in contact with the first surface ofthe template and wherein the coating lies between the first surface andthe solder paste.
 6. The method according to claim 5 wherein thetemplate coating is selected from the group consisting of polyimidetapes and fluorocarbon copolymers.
 7. The method according to claim 6wherein the template coating comprises polyimide tapes.
 8. The methodaccording claim 1 wherein the applying of the solder paste comprisesstencilling the disjoint area of solder paste.
 9. The method accordingto claim 8 wherein the disjoint area is measured prior to the step ofcontacting the top of the solder ball and the solder paste.
 10. Themethod according to claim 9 wherein the disjoint area is measured usinga laser tool.
 11. The method according to claim 1 wherein the disjointarea of solder paste has a volume of at least about 8689 and at mostabout 11,200 cubic mils.
 12. The method according to claim 2 wherein thedisjoint area of solder paste has a volume of 11000 cubic mils.
 13. Themethod according to claim 1 wherein the height of the disjoint area ofpaste is at least about 9 and at most about 16 mils.
 14. The methodaccording to claim 13 wherein the height of the paste is about 12 mils.15. The method according to claim 1 wherein the diameter of the paste isat least about 30 and at most about 34.5 mils.
 16. The method accordingto claim 1 wherein the diameter of the paste is 34 mils.
 17. The methodaccording to claim 1 wherein the reflowing is accomplished at atemperature of at least about 200 and at most about 205° C.
 18. Themethod according to claim 1 wherein the interacting comprises contactingthe solder ball and the solder paste, and wherein the distance from thetop of the solder ball to the first surface of the template is apredefined distance.
 19. The method according to claim 18 wherein thedistance is 7 mils.
 20. The method according to claim 18 furthercomprising at least one standoff situated between the first object andthe template and wherein the standoff is capable of ensuring that thedistance from the top of the solder ball to the first surface of thetemplate is a predefined distance.
 21. The method according to claim 20wherein the distance is 7 mils.
 22. A method of forming a solder ballstructure on a ceramic ball grid array comprising the steps of:a)obtaining a module having a plurality of solder balls having a top and abottom, and the module also having a plurality of BLMs having a top anda bottom, wherein the bottom of the BLM is in electrical communicationwith the module, and wherein the top of the BLM is contacting the bottomof the solder ball, and wherein the BLM comprises 37% Pb and 63% Sn inadmixture, the solder balls comprises 90% Pb and 10% Sn in admixture; b)obtaining a thin, flat, glass plate having a first side and having anon-wettable film coating comprising a polyimide tape attached to thefirst side, and wherein the flat plate is operably associated with aplurality of standoffs capable of ensuring that the distance between theflat plate and the top of the solder balls is 7 mils; c) stenciling aplurality of disjoint areas of solder paste on the first side of thecoated thin, flat glass plate, wherein each of the area corresponds toone solder ball, and wherein the disjoint areas of solder paste have avolume of about 11,000 cubic mils, a height of about 12 mils, and adiameter of about 34 mils, d) contacting the module and the flat platesuch that the standoffs are abutting the module; and e) reflowing to atemperature from about 200 to about 205° C. for at least about 20seconds and at most about 30 seconds while the module is in contact withthe flat plate wherein the high melting point solder ball is coated withthe low melting point solder over substantially all of the surface ofthe high melting point solder ball not in contact with the BLM; f)disengaging the contact between the module and the flat plate.